(1) Field of the Invention
The present invention relates to synthetic rubber compositions of improved tack and adhesion and, more particularly, to styrene-butadiene rubber, or SBR, and carboxylated SBR compositions of improved tack and adhesion. Further, the present invention relates to a process for producing such synthetic rubber compositions, whereby a mixture is formed of from 1% to 25% by dry weight saponified tall oil pitch and from 99% to 75% by dry weight SBR (either carboxylated or uncarboxylated). Fillers can also be included in the mixture as well as other ingredients.
(2) Description of the Prior Art
SBR has a relatively low polarity compared to other synthetic rubbers; and unlike natural rubber, SBR does not develop surface peroxidal activity upon mastication. Therefore, SBR has relatively poor inherent or processed tack. An adhesive is said to possess tack if, under the conditions of application, only light pressure is required to produce a bond sufficiently strong to require work to restore the interface to its original spearated state. Poor tack in SBR can be overcome by adding tackifiers, such as resins or natural rubber. The obvious drawback to using natural rubber, of course, is that any improvement realized is attained at the expense of using the very product which the polymer is intended to replace.
In German Offenlegungschrift No. 2,005,244, Aug. 6, 1970, natural rubber was used in conjunction with SBR latex to improve the tack of flexible, elastic, pressure-sensitive adhesive tapes prepared from polyvinyl chloride as support.
A tire is usually constructed by applying layers of rubber-coated fabric one to another, followed by a breaker strip, cushion, and tread. The layers must possess sufficient surface tack to adhere firmly one to another and to maintain the desired relative position of the various parts prior to vulcanization. Absence of tack in many cases causes difficulty in the building operation. Thus, tack is an important property in the tire-building field. A study published in 33 Moscow Rubber-Chemical Technology, 556-580 (1960) comparing tires of natural rubber; 100% butadiene-styrene rubber; and 100% Na butadiene rubber showed that failure of a tire body depends mainly either on unsatisfactory cohesive properties of the vulcanizate or insufficient adhesion of the layers and is due to hysteresis loss by frictional heat produced during use, causing stripping of the tread.
In tire building and recapping, tire tread compounds are essentially 100% synthetic rubber, most often SBR, which is lacking in building, or "laying up", tack. Therefore, when the tread compound comes out of the extruder, a natural rubber cement is applied on the underside of the tread. Sidewall sections are treated in a similar manner. When the tread is wrapped around the carcass, cement is applied to keep the tread splice in place until the "green" tire is vulcanized. The above-mentioned study determined that use of cements for joining the layers reduces the dynamic bond strength of the plies. Improving the tack of SBR latex would eliminate, or substantially reduce, the need for the relatively expensive natural rubber in tire building and recapping.
It has been known for several years that incorporation of a very small percent of a copolymerizable unsaturated fatty acid monomer with styrene or styrene and butadiene improves the adhesive properties of the resulting polymer (i.e., Netherlands Application No. 6,411,493, Apr. 15, 1965--acrylic acid and fumaric acid and German Offenlegungschrift No. 2,437,365, Feb. 13, 1975--itaconic). Also, methacrylic and crotonic acids are suited to this purpose. In addition to improving adhesion, carboxylation provides reaction sites for cross-linking with curing agents to improve water resistance and improve processing properties. Thus, carboxylated polystyrene or styrene-butadiene rubber latex has been the backbone of many water-based adhesives for some time.
Just as natural rubber is added to SBR to improve its tack, addition of natural rubber latex to the carboxylated latex increases the wet grab (green tack) of the adhesive system. However, due to the high cost of natural rubber latex, the natural latex gradually has been removed from adhesive systems in certain applications, such as the tufted carpet industry where carboxylated SBR, usually highly-filled, is the most important carpet-backing adhesive. The function of the adhesive is to lock the tuft into the primary backing and bind the secondary backing or foam to the carpet underside. With the carboxylated SBR latex alone, however, wet grab to the secondary backing is borderline. If the secondary backing separates from the carpet at any stage of drying, the final bond will never be as strong as if a coherent system were retained throughout the drying cycle. For this reason, the carpet industry, in particular, is interested in more economic methods of increasing the green tack of the carboxylated SBR adhesive, thereby enhancing the final bond of carpet to backing.
The prior art includes many references to improving the properties of synthetic rubber by the incorporation therein of tall oil products. Four main products are usually obtained when refining crude tall oil distillation, namely, fore distillate (5-10%), tall oil fatty acids and a small amount of rosin-containing oils (30-35%), tall oil rosin (30-35%), and as a distillation residue, tall oil pitch (20-30%). Only the second and the third products are considered valuable raw materials and used by the chemical industry. In many distilleries at least the bulk of the fore distillate and the pitch is commonly burned.
In U.S. Pat. No. 2,843,643, demethylated wood tar distillate is added to natural rubber to prevent the cracking of the rubber due to attack by ozone. The patentee states that the invention can be utilized also for preventing ozone cracking in synthetic rubber (column 1, lines 60-71) and names SBR specifically.
In U.S. Pat. No. 3,157,609, pine tar and rosin acid soap are added with an extract of the plant Grindelia to a synthetic rubber polymer as a physical rubber softener, whereby the tack is improved in the resultant polymer.
U.S. Pat. No. 3,632,855 relates to a composition of matter formed by the reaction of one mole of rosin acid with one mole of a fatty olefin epoxide to give a hydroxy ester, useful as a tackifier in SBR rubber. It also teaches the reaction product of the hydroxy ester with a second mole of rosin to give a diester, which is also useful as a tackifier for SBR.
U.S. Pat. No. 3,474,059 teaches that the adhesive properties of elastomeric olefin polymers can be improved substantially by the incorporation therein of a relatively small amount of a tackifier additive comprised of at least two specific rosin-base materials.
In U.S. Pat. No. 3,649,580, the tack of an ethylenepropylene terpolymer rubber was improved by the addition of 1% to 20% tall oil pitch containing 80% to 100% nonvolatile material as a 40.degree. C. to 90.degree. C. solution in rubber processing oil. Thus, dark brown tall oil pitch was distilled in a Pfaulder wiped film evaporator at 275.degree. C. and 0.5-1.0 mm. of mercury pressure until all volatile material was removed to give a tackifier of specific hardness.
U.S. Pat. No. 3,873,482 discloses a process by which commercial tall oil materials, a mixture of fatty acids and resin acids, are pyrolyzed in a hot tube to yield products which when added to a synthetic rubber composition improve its tack.
There have been processes proposed to improve the low quality, semifluid, tarlike, non-distillable tall oil pitch whereby it can be transformed into one or more products suitable for industrial applications.
For example, U.S. Pat. No. 3,926,936 discloses a process for manufacturing fatty acid and rosin, or a mixture of both, plus hard pitch of a new type from tall oil pitch.
U.S. Pat. No. 3,943,117 relates to a process for saponifying tall oil pitch and for hydrolyzing the fatty acid esters found in a tall oil pitch fraction using small amounts of a cationic amine catalyst at specific conditions.
Finally, in 19 Izv. Vyssh. Uchebn. Zaved., Lesn. Zh., 1976, 113-116 (Russ.) the optimum saponification conditions of tall oil pitch, which give the highest yield of phytosterol, were determined by simplex analysis.
The saponified tall oil pitch of this invention is the bottom fraction (pitch) of a crude tall oil distillation which has been at least partially saponified such that the product of the saponification treatment has an acid number above the acid number of the pitch prior to its saponification.
It is theorized that the presence of saponified pitch product in mixture with any remaining unsaponified pitch facilitates the emulsification (or dispersion) of the unsaponified portion. At an acid number of about 70 or above, the saponified pitch is essentially completely water soluble; and for certain applications, a water soluble pitch may be preferred. However, to achieve the improved tackifying properties of this invention, water-miscibility only is required.
The object of the present invention is to provide synthetic rubber compositions with improved tack and adhesion, and, more particularly, to provide sytrene-butadiene rubber and carboxylated SBR compositions of improved tack and adhesion. While the composition of the invention is suitable for any application wherein a sytrene-butadiene rubber with enhanced tack and adhesion properties is desired, it is particularly suited to use an an adhesive in tufted carpet manufacturing (wherein the SBR is carboxylated) and for use in tire manufacturing and retreading (wherein the SBR is uncarboxylated). Other areas of application of this invention would be in paper saturation and synthetic rubber soles for footwear.